Controlling the Morphology of Resorcinol-Formaldehyde-Based Carbon Xerogels by Sol Concentration, Shearing, and Surfactants Chandra S. Sharma, Devendra K. Upadhyay, and Ashutosh Sharma* Department of Chemical Engineering and DST Unit on Nanosciences, Indian Institute of Technology, Kanpur-208016, U.P., India Carbon xerogel microspheres were synthesized by inverse emulsion polymerization of resorcinol with formaldehyde, followed by pyrolysis at 900 °C under nitrogen atmosphere. We have studied the effect of various synthesis parameters, including dilution ratio and hydrophilic-lipophilic balance (HLB) of nonionic surfactants, on the size and morphology of resulting structures. The average particle size of carbon microspheres could be modulated from 1 to 28 μm by varying the dilution ratio over 3 orders of magnitude. Increase in the HLB value and the amount of surfactants produced a variety of dense carbon, but open-architecture fractal- like structures. Three different methods of stirring, namely, magnetic, mechanical, and ultrasonication were applied during the inverse emulsification to produce carbon xerogel microstructures. Formation of a wide spectrum of nonporous carbon particle morphologies, including the highly branched, hierarchical microparticles, by tuning the synthesis conditions may have potential applications in printing technology, controlled drug delivery, biosensors, and carbon-based microelectromechanical systems (C-MEMS) including bio-MEMS. Introduction Organic gels have been a subject of interest for almost the last two decades because of their unique physical, chemical, and electrochemical properties. 1-17 Although new precursors and solvents have been used in the synthesis of carbon gels recently, 18-20 the sol-gel polycondensation route of resorcinol- formaldehyde (RF) is still the most widely used since their introduction by Pekala. 1,2 In this process, resorcinol reacts with formaldehyde under alkaline conditions in two steps. 1,9 The first step is the formation of hydroxymethyl derivatives by an addition reaction, and the next step is condensation of these derivatives to form methylene and methylene ether. 1,9 These intermediates further react to form highly cross-linked clusters of RF gel that are pyrolyzed to form carbon gels. These carbon gels are used in a variety of applications including electrode materials in rechargeable batteries and supercapacitors, etc. 2-16 The morphology of these carbon gel particles is one of the most important single factors in considering their potential for a specific application. Various kinds of morphologies in these carbon gels, for example, porous texture, microspheres, thin film, granule, and honeycomb, etc. have been reported. 1-20 Recently, dense carbon microspheres have been used as a template for the preparation of oxide hollow spheres to be used in controlled drug delivery. 21,22 There are other potential applications of dense carbon microspheres such as studying the interactions of carbon with a variety of surfaces using a microbead attached to an AFM tip. The interaction between such model particles and fabric substrates is important for detergency as it provides fundamental understanding of the forces and their manipulation to aid their removal from the surface. As reported in our recent work, 14 RF xerogel microparticles can be synthesized by inverse emulsion polymerization followed by air-drying. In this ap- proach, RF sol is added slowly in the presence of a surfactant to form a dispersed colloidal solution. These particles were pyrolyzed at a high temperature in an inert atmosphere to obtain carbon microparticles. Sol-gel synthesis variables, method of drying, and pyrolysis conditions play a major role in determining the morphologies of carbon gels derived from the RF sol. Recently, we have shown that by tuning some of these synthesis parameters, for example, the amount of catalyst in the RF sol and the concentration of surfactant employed in the microemul- sion, dense carbon structures lacking micropores but with a high external surface area resulting from their fractal-like morphology are formed. 14 Such structures may find applications in resonance- based solar cells 23 and carbon-based microelectromechanical systems (C-MEMS), including microbatteries. 14,24,25 Dense xerogels lacking micropores appear to be specially promising anode materials because intercalation of Li ions in micropores is more irreversible leading to a decline in the reversible capacity of the electrode material. Indeed, it has been shown that the reversible capacity of mesoporous carbon microbeads (MCMB) declines with an increase in its specific surface area, much of which is contributed by micro/mesopores. 25 Similar consider- ation also holds for aerogels. 1 Also, these microstructures have the potential to serve as a platform for cell manipulations and growth because of the biocompatibility of carbon coupled with its easy functionalization and hydrophobicity/hydrophilicity control. 26 Various method of drying may be chosen after sol-gel polymerization to obtain different levels of porosity in these carbon gels. Organic xerogels are made by removing the solvent from the gel structure by drying conventionally with nitrogen or air under normal conditions which is followed by pyrolysis to yield RF-based carbogel. These xerogels are generally dense because the capillary pressure of the liquid inside a pore leads to the collapse of the gel-network during oven drying. 9,10,14,15 However, there are few reports to synthesize the porous carbon xerogels also by changing the initial pH of the RF sol. 10-12 The specific aim of the present work is to develop a better control on the size and morphology of dense RF-derived carbon xerogel particles. We have investigated the role of diluent concentration (water in the present case) and the method of stirring, both of which affect the particle morphologies to a great extent as shown here. Other than this, choice of a surfactant with a particular HLB value to yield nonporous carbon xerogel microstructures is also studied here, which has not been discussed in literature to the best of our knowledge. Finally, * To whom correspondence should be addressed. E-mail: ashutos@ iitk.ac.in. Tel.: +91-512-259 7026. Fax: +91-512-259 0104. Ind. Eng. Chem. Res. 2009, 48, 8030–8036 8030 10.1021/ie900359w CCC: $40.75  2009 American Chemical Society Published on Web 05/11/2009